Without limiting the scope of the present invention, its background is described in connection with transmitting downhole data to the surface during measurements while drilling (MWD), as an example. It should be noted that the principles of the present invention are applicable not only during drilling, but throughout the life of a wellbore including, but not limited to, during logging, testing, completing and producing the well.
Heretofore, in this field, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The utilization of MWD with real time data transmission provides substantial benefits during a drilling operation. For example, continuous monitoring of downhole conditions allows for an immediate response to potential well control problems and improves mud programs.
Measurement of parameters such as bit weight, torque, wear and bearing condition in real time provides for a more efficient drilling operation. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques.
At present, there are four major categories of telemetry systems that have been used in an attempt to provide real time data from the vicinity of the drill bit to the surface, namely mud pressure pulses, insulated conductors, acoustics and electromagnetic waves.
In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar near the bit. This type of system typically transmits at 1 bit per second as the pressure pulse travels up the mud column at or near the velocity of sound in the mud. It has been found, however, that the rate of transmission of measurements is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of mud flow.
Insulated conductors, or hard wire connection from the bit to the surface, is an alternative method for establishing downhole communications. This type of system is capable of a high data rate and two way communications are possible. It has been found, however, that this type of system requires a special drill pipe and special tool joint connectors which substantially increase the cost of a drilling operation. Also, these systems are prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string.
Acoustic systems have provided a third alternative. Typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission.
The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. A current carrying downhole data is input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string. An electromagnetic receiver is inserted into the ground at the surface where the electromagnetic data is picked up and recorded. It has been found, however, that it is necessary to have an electrically insulated subassembly in the drill string in order to generate the electromagnetic waves. Conventional electromagnetic systems have used dielectric materials such as plastic resins between the threads of drill pipe joints or within sections of drill pipe. It has been found, however, that these dielectric materials may be unable to withstand the extreme tensile, compressive and torsional loading that occurs during a drilling operation.
Therefore, a need has arisen for a gap subassembly that electrically isolates portions of a drill string and that is capable of being used for telemetering real time data from the vicinity of the drill bit in a deep or noisy well using electromagnetic waves to carry the information. A need has also arisen for a gap subassembly that is capable of withstanding the extreme tensile, compressive and torsional loading that occurs during a drilling operation.